This invention relates to an exposure apparatus for copying machines.
In a typical copying machine a photoconductive member such as a drum is uniformly charged and exposed with a light image from an original document to form an electrostatic image. A developer is applied to the drum to develop the electrostatic image into a visible toner image which is transferred and fixed to a sheet of paper.
It is well known in the electrostatic copying machine field that the charge acceptance of the photoconductive member changes with the temperature of the member. As the charge acceptance varies, the quality of the copy image is affected.
For example, FIG. 1 shows a graph of charge acceptance versus temperature of the photoconductive member for constant luminance. This data was obtained by charging an amorphous selenium photoconductive member with a corona charger under a constant voltage condition. The ordinate shows the charging potential (V), while the abscissa shows the temperature (T.degree.C.) of the atmosphere surrounding the photoconductive member. The shaded portion is the permissible charging potential region for quality reproduction. It can be seen from this data that the charging potential (V) decreases with temperatures above 30.degree. C.; moreover, temperatures in the region above 40.degree. C. are outside the permissible charging potential range. In other words, with temperature conditions above 40.degree. C. and constant luminance of exposing light, the charging potential (V) corresponds to an excessive exposure; as a result, the copy image quality will be degraded. This is attributable to the fact that the electrical resistance of the photoconductive member is reduced with an increase in temperature; consequently the acceptance of the member for holding electrostatic charges is reduced. It is necessary, therefore, to control the luminance from the exposure lamp which is focused on the photoconductive member.
Japanese Patent Publication (Kokoku) No. 54-22305 (M. Nakaguchi; Aug. 6, 1979) discloses an exposure control device utilized in the prior art containing a thermistor for detecting the temperature of the photoconductive member. A signal representing the detected temperature is then utilized to control the exposure lamp or the size of the aperture which passes the light beam from the exposure lamp. Such prior art devices, however, have many disadvantages such as unreliability, complicated construction and extreme high cost.